The present invention relates, in general, to surfacing methods and apparatus, and, more particularly, to improvements in such surfacing methods and apparatus.
Surfacing is the deposition of filler metal on the surface of a base metal. Its purpose is to provide the properties or dimensions necessary to meet a given service requirement. There are several types of surfacing. The major categories are cladding, hard facing and build-up.
The desired properties are corrosion resistance, wear resistance, dimensional control and metallurgical needs.
Surfacing is often used in the field of repair as well as the initial manufacturing.
Among the main economic and technical objectives of any surfacing process are: (1) to maintain low dilution; and (2) to achieve high deposition rates, particularly with attractive usability characteristics. Low dilution allows the deposit to perform at its maximum potential and to make the application of additional layers unnecessary. High deposition rates are highly desirable from an obvious economic viewpoint. An example of a very high deposition rate would be a rate greater than 40 lbs. per hour.
It is very difficult to achieve the first two objectives simultaneously, as high electrode melting rates are generally accomplished by the use of high currents which invariably lead to high dilution. Consequently, many of the high deposition rate processes suffer from high dilution and penetration. Also, the high heat input required to realize high deposition rates limits the processes considerably. Specific precautions must be taken to reduce the workpiece temperature to prevent overheating of the workpiece and also to limit dilution and penetration. There are state of the art methods that attempt to overcome these problems. One method is to use a long electrical stickout to minimize penetration, however, arc instability, excessive spatter, difficult arc starts limits the usefulness of this approach. Another method is the use of strip electrodes-submerged arc welding. While high deposition rates are achieved it is generally at the expense of alloy selection since hard facing strip electrodes cannot be economically or practically fabricated.
Surfacing is distinguished from joining in several important areas such as dilution control. In fact, probably the single biggest difference between welding a joint and surfacing is that the former penetration is normally desirable whereas in the latter penetration is frequently undesirable.
From a metallurgical point of view, the compositions and properties of surfacing deposits are strongly influenced by the extent of dilution. An example of a joining method is disclosed in U.S. Pat. No. 4,246,463.
Thus, there is a need for a surfacing process which combines high deposition rate, low dilution, effective process control, and adequate alloy selection.